Synergy Study On Hydrothermal Conversion Of Cellulose Coupled With Heat And Mass Transfer

To realize green and efficient transformation of lignocellulosic biomass into value added chemicals and fuels,hydrothermal conversion technology could be a promising thermochemical conversion technology.Essentially,biomass hydrothermal conversion is the process that biomass components are transformed into target products through a series of multi-scale physical and chemical changes.A full understanding of the coupling mechanism between physical and chemical processes is fundamental to improve the reaction efficiency and optimize the reaction system of biomass hydrothermal conversion.Unfortunately,studies in biomass hydrothermal conversion mainly focused on the chemical processes such as catalyst development,reaction path design and reaction mechanism exploration,while the investigation on physical processes(multiphase flow,component transport and heat transfer etc.),especially on the coupling mechanism between multi-scale physical and chemical processes,was relatively few to this date.The present thesis is devoted to study the coupling mechanism of multi-scale and multi-physicochemical processes and its influence on the reaction in biomass hydrothermal conversion.The concept of physicochemical sub-process set was proposed,in which the complex reaction networks in biomass hydrothermal conversion are summarized and divided into unified physicochemical sub processes.Numerical models with multi-scale and multi-physicochemical process coupling were constructed based on lattice Boltzmann methd(LBM).As a typical representative,the reaction path"cellulose →glucose →HMF →LA → GVL" which includes the main physicochemical processes in biomass hydrothermal conversion,was investigate systematically.The following is a sumerization of this thesis:(1)A numerical model for the hydrothermal depolymerization of cellulose in acidic environment was developed.The dissolution process of cellulose was accurately predicted,and the evolution mechanism of biomass particle structure was brought to light.Combination the reaction efficiency and raw consumption,the hydrolysis reaction efficiency factor was proposed and comprehensive evaluation of the reaction system conducted based on this parameter.Furthermore,the inhibition mechanism of lignin on the hydrothermal depolymerization process of cellulose was revealed.(2)The fluid flow,multi-components transport and geometric characteristics of catalyst bed coupled with the dehydration of glucose into HMF was studied by a porous media fixed bed numerical model.The connotation of component diffusion and mass transfer in solvent effect was analyzed.The results demonstrated that the diffusion limitation effect of the intermediates is conducive to the formation of target products.In particular,it was found that for the dehydration of glucose,the catalyst bed shows obvious regional characteristics along the flow field direction,namely:inlet section,reaction rate control region,Reynolds number influence area and the zone of low catalytic efficiency.The length of each region along the velocity direction does not depend on the Reynolds number.This regional characteristics can provide an important guidance for the design of the reactor.(3)A coupled reaction-transport model for the simulation of coking phenomenon in biomass hydrothermal conversion was constructed.The coking characteristics of glucose conversion to levulinic acid(LA)in Fe/HY zeolite catalyst were studied.The coupling laws of catalyst particle size,porous medium flow,heat transfer,chemical reaction and coking process were revealed.The inducing mechanism of larger catalyst particles on coking process was clarified.In addition,based on the numerical simulation results and the secondary analysis of the experimental data in the literature,the limitations of the present coking reaction kinetic model in the high temperature were found.(4)A pore-scale multiphase multi-component reactive transport model was developed to simulate the hydrogenation of LA to GVL over Ru/C catalyst in a gasliquid-solid reaction system.Reaction and transport processes of LA and H2 within the catalyst particle were simulated,and the LA mass transfer limitation effect "LA effect"was found to be caused by low LA concentration.Furthermore,an impact factor of hydrogen pressure was proposed to evaluate the effect on LA hydrogenation quantitatively.Dominant role of hydrogen mass transfer at gas-liquid interface was demonstrated.It was proposed that hollow spheres or coating catalyst would present a better synergism with LA hydrogenation to GVL.(5)The porous catalyst particle models with a series of active coating thickness were reconstructed numerically to reveal the stage characteristics of the reaction and the phenomenon of hydrogen penetration depth.The kinetic control and mass transfer control characteristics of LA hydrogenation process were analyzed.The results shown that the utilization of coated catalyst can reduce the amount of catalyst by 30%when the reaction performance is almost unchanged,proving the synergistic effect of coated catalyst on LA hydrogenation.